Fluid bed cooling system for polymer processing

ABSTRACT

A system for the solid phase polymerization of polymers wherein cold amorphous polymer is introduced to a crystallizer and heated to crystallize the polymer, the crystallized polymer is discharged to a reactor to achieve polymerization of the polymer, and the hot polymer product of the reactor is discharged to a fluid bed cooler for cooling of the polymerized product. The fluid bed cooler includes an inlet for the hot polymer and an inlet for the cooling gas, and the cooled polymer and the heated gas are discharged from the cooler after contact of the gas with the polymer. The cooler includes a bed portion through which the polymer moves while in contact with the cooling gas, and an upper chamber which collects the heated gas. The temperature of the polymer gradually decreases from the location of the inlet for the polymer to the location of the discharge for the polymer, and the gas in the upper chamber is hotter in the area of the upper chamber adjacent the inlet for the hot polymer than in the area of the upper chamber adjacent the discharge location of the cooled polymer. At least two outlets are defined by the upper chamber and a weir baffle is used to separate an upper chamber zone containing the hotter gas from the balance of the upper chamber. Hotter gas is then discharged to the reactor through an outlet communicating with the upper chamber zone containing the hotter gas.

This is a divisional of application Ser. No. 08/496,762 filed on Jun.29, 1995, now, U.S. Pat. No. 5,536,880.

BACKGROUND OF THE INVENTION

This invention relates to polymer processing systems which treatamorphous polyester to achieve crystallization followed by solid phasepolymerization, followed by cooling. An example of such a system isfound in Herron Patent No. 4,161,578.

In systems of the type contemplated by this invention, cold amorphouspolymer material is fed to a crystallizer and, after substantial heatinput, hot product is discharged. The particular apparatus for achievingthe crystallization may comprise an indirect heat supply unit or a fluidbed. The SOLIDAIRE® or TORUSDISC® equipment manufactured by HosokawaBepex Corporation of Minneapolis, Minn. is an example of an indirectheat supply unit which may be utilized for achieving thecrystallization. In such a system, steam or other heated fluid is passedthrough rotors and/or jackets which are in contact with the agitatedpolymer material. The polymer is thereby heated to the necessarytemperature for achieving the crystallization reaction.

In a fluid bed system, for example units manufactured by Hosokawa Bepex,heated air is brought into contact with the polymer material in order toimprove heat transfer and to achieve the temperatures required for thecrystallization reaction.

In systems of the type described, the crystallized material istransferred to a reactor for achieving polymerization. Subsequent to thepolymerization, the material is transferred to a cooler which may alsocomprise, for example, a TORUSDISC® or fluid bed system. Where a fluidbed cooler is employed, it has been the practice to achieve the cooling,at least in part, by contacting the hot material with cool gas which maybe air or some other gas such as nitrogen. As a result, the gas isheated considerably and may then be recirculated only if first cooled.The expense of cooling is, however, only warranted where nitrogen orsome other more expensive gas is being used.

SUMMARY OF THE INVENTION

This invention provides a system for polymer processing wherein greatlyimproved efficiencies are achieved in the operation of the coolingportion of the cycle. In particular, the system constitutes a processand apparatus for recirculation of gases used in a fluid bed coolingoperation which greatly improves the efficiency of this operation.

The system of the invention comprises, in particular, an arrangementwherein solid phase polymerization is achieved by introducing coldamorphous polymer to a crystallizer to crystallize the polymer. Thecrystallized polymer is then discharged to a reactor for further heatingto achieve polymerization of the polymer, and the hot polymer product ofthe reactor is discharged to a fluid bed cooler for cooling of thepolymerized product. The fluid bed cooler includes an inlet for the hotpolymer and an inlet for the cooling gas, and the cooled polymer and theheated gas are discharged from the cooler after contact of the gas withthe polymer. The cooler includes a bed portion in which the polymermoves while in contact with the cooling gas, and an upper chamber whichcollects the heated gas. The temperature of the polymer graduallydecreases from the location of the inlet for the polymer to the locationof the discharge for the polymer, and the gas in the upper chamber ishotter in the area of the upper chamber adjacent to the inlet for thehot polymer than in the area of the upper chamber adjacent to thedischarge location of the cooled polymer. At least two gas outlets aredefined by the upper chamber and a weir baffle is used to separate anupper chamber zone containing the hotter gas from the balance of theupper chamber. Hotter gas is then discharged through an outletcommunicating with the upper chamber zone containing the hotter gas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a prior art system for solid phasepolymerization wherein a fluid bed cooler is employed;

FIG. 2 is a schematic illustration of a system for solid phasepolymerization wherein the fluid bed cooler is characterized by thefeatures of this invention;

FIG. 3 is a perspective view of a fluid bed cooler suitable for use inthe practice of the invention;

FIG. 4 is a schematic side elevational view of a fluid bed cooler of thetype employed in the practice of the invention;

FIG. 5 is a schematic elevational view of a weir structure for thecooler; and,

FIG. 6 is a schematic elevational view of an alternative form of weirstructure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The polymerization system shown in FIG. 1 comprises a feed inlet 10 forreceiving cold amorphous polymer material and for delivering thematerial to fluid bed crystallizer 12. Hot gases are introduced throughconduits 13 and 15 to achieve heating and agitation of the polymer whilethe polymer is being moved from the input end to the output end of thecrystallizer.

The fluid bed unit 12 is preferably of the type manufactured by HosokawaBepex Corporation. Air or other gas is introduced into the unit 12 forachieving fluidization and heating of the polymer and this results inagitation and progression of the polymer through the unit from the entryend to the discharge end. The agitation achieved by the fluidizingoperation insures heat transfer to the polymer product whereby theproduct will be heated to the desired temperature for crystallization.

Stripper column 14 receives the crystallized material from the outputend of the crystallizer 12. In conventional fashion, hot air, nitrogen,or other gases are introduced to the hopper/dryer through conduit 16.These gases, as well as the hot gases from the crystallizer 12 arepassed through conduit 18 to conventional cleaning and filteringequipment 20 and 22 (also used in other parts of the system) whereby thegases may be recirculated after passing through heaters 24 and 26positioned at the ends of conduits 13 and 15. The unit 20 may comprise acyclone separator and the unit 22 a swing cartridge filter.

Preheater/post crystallizer/annealer unit 28 is positioned at the outletof the stripper column 14. This preferably comprises a TORUSDISC® orTHERMASCREW® unit of the type manufactured by Hosokawa BepexCorporation. Such units serve to further heat the crystallized materialwhile agitating the material to achieve post-crystallization andannealing of the polymer product.

The TORUSDISC unit, for example, consists of a stationary horizontalvessel containing a tubular rotor with vertically-mounted double-walleddiscs. These discs provide approximately 85% of the total heatingsurface. Other heating surfaces are the rotor shaft and the inner wallof the jacketed vessel trough. As a consideration of literatureillustrating the product will reveal, inlet 31 and outlet 33 areprovided so that hot gases are adapted to be circulated through theunit. These gases are also adapted to be recirculated to the fluid bedcrystallizer 12.

Material is delivered from the unit 28 and to hopper/reactor 32 wherethe solid phase polymerization takes place in a conventional fashion.Material exiting from the hopper/reactor is then delivered to inlet 29of fluid bed cooler 34 and, after cooling, is discharged from outlet 35.

In the prior art system shown in FIG. 1, hot gases are introduced to thereactor 32 from conduit 40 and the gases exit through conduit 42. Aftercleaning, the gases, which still retain substantial heat, are fed tostripper column 14 and unit 28. Some of the gas from line 15, prior toreaching heater 26, is fed through conduit 44 to the fluid bed cooler. Adehumidifier 46 is interposed in conduit 44, and a plenum chamber 37 ofcooler 34 receives the gas from conduit 44.

In a typical prior art operation, the gas entering fluid bed cooler 34is at 110° F. and the gas in conduit 48 exiting from the cooler is at195° F. As illustrated, the gas entering the fluid bed cooler comprisesa combination of gas from conduit 44 and conduit 48, and since thelatter gas is initially too hot, some cooling must be achieved. This canbe done only by insuring that the gas in conduit 44 is sufficiently coolto bring the temperature of the gas from conduit 48 down sufficiently,or cooling means 50 must be employed to cool the combination beforeentry into the cooler 34.

The example of a prior art system shown also involves the need for gasentering reactor 32 through conduit 40 at a temperature of 410° F. Thisgas consists of recirculated gas from conduit 48 and make-up gas frominlet 52. To achieve the necessary temperature level, the gas must beheated considerably by means of heater 54 before entering the reactor 32from conduit 40. Blower 51 and damper 53 are used for controlling thegas flow in line 40.

In a typical system, 110,000 pounds per hour of gas at 110° F. are fedto fluid bed cooler 34 and an essentially equal amount exits from thecooler at 195° F. The reactor 32 requires an input of 20,700 pounds perhour and all of this gas, plus any make-up gas must be heated to 410° F.It has been determined that 1.113 million BTU's per hour is required toachieve this heat input.

The balance of the gas exiting from cooler 34 into conduit 48 (89,300pounds per hour) is supplemented by 20,700 pounds per hour of gas at110° F. from conduit 44. Considerable additional cooling is thereforerequired to reduce the larger amount of gas from 195° F. to thenecessary input temperature of 110° F. Specifically, 1.898 million BTU'sper hour is required to be removed in the cooling means 50 to achievethe cooling.

FIG. 2 illustrates a system generally comparable to that of FIG. 1 fromthe standpoint of the crystallizer 12, unit 28 and reactor 32. Thesystem differs with respect to the structure and operation of the fluidbed cooler 60 used in the system.

In particular, the fluid bed cooler 60 shown in greater detail in FIG. 3comprises a housing 62 defining inlet 64 and outlet 66 for the polymerbeing treated. The polymer is moved over the perforated screen or bubblecap-type gas distributor 68 and cool gas is delivered from plenum 61through the perforations for contact with the hot polymer whereby thepolymer is agitated and cooled as it passes through the cooler.

As is illustrated by arrows 70, the polymer is typically directed aroundbaffle plates 72 to minimize short-circuitry and backflow of solids andmaximize opportunity for uniform exposure of all of the polymer granulesto the cooling gas. In addition, and in accordance with this invention,FIG. 4 illustrates an underflow weir 74 is provided for dividing thecooler housing into separate chambers 76 and 78.

The underflow weir 74 extends to the top of the cooler. Separate gasexhausts 84 and 86 are provided for chambers 76 and 78 and the weirtherefore operates to divide the gas exiting the chamber. Specifically,gas bubbling through the polymer in the chamber 76 will be dischargedthrough exhaust 84 and the balance of the gas will be discharged fromchamber 78 through exhaust 86. Polymer enters cooler 60 through inlet 88and is discharged at outlet 90. The polymer thus enters chamber 76 andsince it has the highest temperature at this time, the gas dischargedfrom chamber 76 is quite hot. On the other hand, the polymer moving pastweir 74 is considerably cooler and therefore the gas discharged fromchamber 78 is at a lower temperature.

The system of the invention utilizes this difference in gas dischargetemperatures to achieve improved operating efficiency. Thus, the gasfrom exhaust 84 is passed by means of blower 91 and damper 93 throughconduit 92 directly to reactor 32. Heater 54 raises the temperature ofthis gas (as well as that of any necessary make up gas) to the degreerequired but this is considerably less of a burden than with aconventional system.

The gas exhausted from chamber 78 is, on the other hand, deliveredthrough conduit 94 for return to fluid bed cooler 60. This gas iscombined with gas from line 44 and is passed through cooling means 50 toachieve the desired temperature level. Although a cooling means isrequired, the burden is considerably less than required in aconventional system.

In an operation of the same type as described with the operation of FIG.1, 20,700 pounds per hour of gas exits from exhaust 84 of the chamber 76at 275° F. Only 0.699 million BTU's per hour, a 37% reduction whencompared with the conventional system, are required to raise thetemperature to the level of 410° F. required for the reactor 32.

The amount of gas obtained from chamber 76 is calculated to correspondto the input needed for reactor 32. In a typical system, the cooler 34may have a bed of 70 square feet and the chamber 76 may be formed over a15 square foot area. The amount of hot gas removed may, of course, bevaried depending on the position of the weir, and any suitablemechanical means, such as a guide slide for the weir combined with arack and pinion arrangement or piston and cylinder, may be used for weiradjustment. The amount of gas exhausted may also be controlled bysetting of blower dampers.

Gas at a temperature of 175° F. is discharged through exhaust 86 at89,300 pounds per hour. Only 1,451 million BTU's per hour are requiredto be removed in the coolng means 50 to lower the temperature of thisgas (combined with 20,700 pounds per hour from line 44) to the 110° F.level required for the fluid bed cooler 60. This constitutes a 24%reduction when compared with the conventional system described.

FIGS. 5 and 6 illustrate examples of specific weir arrangementsoperating in accordance with the invention. As illustrated, the bottomedge of the weir is spaced from the perforated gas distributor 68 topermit underflow passage of polymer from one chamber to the other. Theweir 74' of FIG. 6 includes only a partial opening 75 which controls theflow rate of the polymer. Further control can be achieved by usingremovable bars or the like which would permit variations in the size ofthe openings illustrated.

The method and apparatus of the invention is particularly suited for usewith systems employing nitrogen gas since it is most desirable torecycle this gas to minimize operating costs. Even where air is used,however, the system allows for recovery of significant heat values fromthe air removed from chamber 76. The air from chamber 78 may simply bedisposed of since the heat value is not significant and atmospheric airmay be more efficiently used for mixing with air from line 44.

It will be understood that changes and modifications in the embodimentsdescribed may be made without departing from the spirit of the inventionparticularly as defined in the following claims.

That which is claimed is:
 1. In an apparatus for the solid phasepolymerization of polymers wherein a cold amorphous polymer isintroduced to one or more crystallizers and heated to crystallize thepolymer, the crystallized polymer is discharged to a reactor forpolymerization of the polymer, and the hot polymer product of thereactor is discharged to a fluid bed cooler for cooling of thepolymerized product, said fluid bed cooler including an inlet for thehot polymer and an inlet for the cooling gas, and means for dischargingthe cooled polymer and the heated gas from the cooler after contact ofthe gas with the polymer, said cooler including a bed portion in whichthe polymer moves while in contact with the cooling gas, and an upperchamber collecting heated gas, the temperature of the polymer graduallydecreasing from the location of the inlet for the polymer to thelocation of the discharge for the polymer, and the gas in the upperchamber being hotter in the area of the upper chamber adjacent the inletfor the hot polymer than in the area of the upper chamber adjacent themeans for discharging the cooled polymer, the improvement wherein saidupper chamber defines at least two outlets to serve as said means fordischarging the heated gas, means separating an upper chamber zonecontaining the hotter gas from the balance of the upper chamber, andwherein one of said upper chamber outlets communicates with the upperchamber zone whereby the hotter gas is recovered through said oneoutlet.
 2. An apparatus according to claim 1 including a gas distributorcomprising a perforated screen or bubble cap-type, and including aplenum chamber extending beneath said gas distributor through which saidcooling gas enters the cooler, said cooling gas moving upwardly throughsaid gas distributor and through the polymer supported thereon and intosaid upper chamber, and a weir extending downwardly in said upperchamber to a position spaced upwardly from said gas distributor topermit movement of polymer over the gas distributor while said weirserves as said means for separating said zone of said upper chamber fromthe balance of the upper chamber.
 3. An apparatus according to claim 2wherein hot gas is used to provide at least part of the heat input forsaid reactor, and including a conduit extending from said one outlet tosaid reactor and means at said reactor for using the heat value of saidhotter gas to assist in heating the gas entering the reactor.
 4. Anapparatus according to claim 3 including means for heating said hottergas prior to delivery to said reactor.
 5. An apparatus according toclaim 3 including a conduit extending from another of said outlets tosaid inlet for said cooling gas whereby the balance of said gas isrecirculated to said cooler.
 6. An apparatus according to claim 5including means for cooling said heated gas prior to delivery to saidinlet for said cooler.